(CNN) -- Thirty to 40 ships -- including several passenger ships -- were stuck Thursday in ice off the coast of Sweden, said a spokesman for the Maritime Search and Rescue Center in Gothenburg, Sweden.

The area of the Baltic Sea worst hit by the ice were the waters bounded by mainland Sweden, the Stockholm archipelago and the Finnish island of Aland, said Tommy Gardebring, press officer with the Swedish Maritime Administration.

The center identified one of the passenger ships as the Amorella, with 753 passengers and 190 crew members.

Several passenger vessels from Viking Line were stuck, he said. One of them had been freed.

"It has been a lot colder than normal in the southern parts of the Baltic sea, but in the north all is normal with normal levels of ice," Gardebring said. "However, in the worst-affected areas, the ice breakers that normally operate haven't been able to cope with the ice, which is why we are sending additional ice breakers."

The extra help was expected to arrive around midnight (6 p.m. ET), he said.

"There was never any danger for the safety of the vessels, but we have increased our preparedness, just in case, since the ice puts a lot of pressure on the sides of the ships," Gardebring said.

He predicted that most of the ships would be freed by Friday.

CNN's Per Nyberg and Claudia Rebaza contributed to this story from London, England